Ink jet printheads are sandwich structures having a top plate, an intermediate photoresist layer, and a bottom plate. The photoresist layer is imaged to define ink passageways, which carry ink to outlet nozzles mounted on the top plate. The bottom plate typically is a thermally stable substrate, such as a silicon wafer, that bears microcircuits. Microresistors are mounted on the bottom plate, projecting into the liquid pathways in the photoresist layer, in alignment with the ink nozzles. At computer command, the resistors superheat nearby ink, creating a steam bubble that forces ink droplets out the nozzles.
The top plate frequently has a noble metal surface (typically gold) that is not wetted by the aqueous ink to minimize accumulation of residual ink during the firing cycle since ink accumulation may interfere with the design trajectory of the ink droplets. While noble metals may be desired for this purpose, it is difficult to achieve a durable bond between the noble metal and the photoresist layer during the millions of firing cycles the pen will experience during its lifetime.